a) Field of the Invention
The invention relates to the adjustment of pulse length in diode-pumped, Q-switched, solid state laser oscillators with variable oscillator output, particularly fundamental mode lasers with a high output power of more than 10 W.
b) Description of the Related Art
For many industrial or medical applications, it is advantageous to vary the pulse lengths and the pulse energy of a laser so as to be able to control its action in a deliberate manner. Pulse lengths in the range of several hundred ns to several μs at pulse repetition rates between 10 kHz and 200 kHz are particularly relevant. At present, the preferred medical application is in ophthalmology and the preferred industrial application is in laser separation of chips on silicon wafers or GaAs wafers. However, there is also an urgent need in the field of materials processing, e.g., for machining ceramics and diamonds, laser honing and laser drilling, although particularly short pulses below 10 ns are required in this case.
Many existing embodiment forms of continuously pumped acousto-optic, Q-switched, solid state laser oscillators, which primarily comprise lamp-pumped or transverse diode-pumped Nd:YAG rod lasers, typically achieve pulse lengths from 50 ns to 200 ns. The large pulse lengths can be achieved at low pump output and only at a high pulse repetition rate, but the short pulse lengths can be achieved only at a low pulse repetition rate. Pulse lengths greater than 200 ns cannot feasibly be produced in technical respects with these constructions because of the difficulty in achieving the transverse fundamental mode operation and due to a low pulse stability. Further, lasers of this kind can also only operate within a very limited parameter window.
While particularly high average outputs are achieved by end-pumped Nd:YVO4 laser oscillators at repetition rates of 100 kHz, it is also the case here that the pulse length at a given cavity configuration (L, V, R, Pth) can only be varied by changing the pump output Pp and, because of the correlation between pulse length and pulse energy, a drastic reduction in output power results when the pulse length is increased. This relationship is described in R. Iffländer, Solid state Lasers for Materials Processing, Springer Series in Optical Sciences, Springer Verlag, Berlin (2001), according to which the pulse length can be calculated from the parameters of pump output Pp, pump output at the laser threshold Pth, out-coupling coefficient R, loss factor V, and cavity length L by the following formula:
  τ  =            L      c        ·                            P          p                          -                                    ln              ⁡                              (                                  V                  ⁢                                      R                                                  )                                      ⁡                          [                                                P                  p                                -                                  P                  th                                -                                                      P                    th                                    ⁢                                      ln                    ⁡                                          (                                                                        P                          p                                                /                                                  P                          th                                                                    )                                                                                  ]                                          .      
Further, the change in the intra-cavity output due to the variation in pump beam output generally leads to a change in the thermal lens of the laser crystal, so that the beam parameters of the coupled-out beam also change. This effect is troublesome in many applications and, in particular, an anisotropically acting thermal lens leads to asymmetry in the beam profile.
A change in the repetition rate of the laser is also directly connected to a change in the pulse duration and output power. The pulse duration and the average output decrease at lower repetition rates.
Further, it is known from DE 199 58 566 A1 and DE 199 27 918 A1 to achieve a variation in pulse length in diode-pumped lasers with intra-cavity frequency doubling for medical applications by means of controlling the Q modulation.
It is disadvantageous that additional oscillator losses caused by the disruption of the pulse buildup lead to poor efficiency and that the highly nonlinear dynamics due to the exponential pulse rise result in a complicated solution with respect to control engineering.